(1) Field of the Invention
This invention relates to the control of jig separators used for the beneficiation of minerals. In particular, the invention is directed to an apparatus for measuring the properties of the jig bed. The information derived from the measurements can be used to provide a continuous control signal to improve the operating efficiency of the jig separators by better regulation of the jig operating parameters.
Throughout the specification the term "minerals" should be employed to include such material as coal, tin ores, gold ores, iron ores, manganese ores and such other valuable materials as can be separated from less valuable materials by gravity concentration. The term "jig" is to be interpreted to means any device using a pulsating fluid to produce stratification according to particle specific gravity in a bed of broken mineral. In usual circumstances the jig treats a continuous flow of mineral and is provided with means for continuous or intermittent discharge of the lower specific gravity and higher specific gravity fractions of the mineral mixture.
(2) Prior Art
The accepted principles of jig operation are described by Wills (B. A. Wills, Mineral Processing Technology, 2nd Edition, Pergamon Press, 1981). Gaudin (A. M. Gaudin, Principles of Mineral Dressing, McGraw Hill, 1939) also discusses the physics of jig operation and means of control of discharge of dense material from jigs.
There are two requirements for efficient jig operation, namely (i) control of heavy product discharge from the jig, and (ii) control of the stratification of the mineral bed in the jig. The term stratification generally refers to the variation in particle density as a function of vertical position in the jig bed in the compacted or closed state. Assuming that the discharge of the dense material is correctly performed, the separation effected by the jig will be more efficient if the stratification is such that the dense mineral and less dense mineral components are present in distinct layers, facilitating discharge of either layer from the jig. If more dense, material is discharged at too high a rate from the bed in a jig compartment, the stratification profile will be altered and it will become impossible to maintain either the desired separation or the efficiency of separation. The desired separation in a jig compartment can be quantitatively described by the jig separation specific gravity SG.sub.50. SG.sub.50 is the density of those mineral particles which are recovered at equal mass flow rates in both the dense and less dense product streams from the compartment.
Various means of regulation of SG.sub.50 are known. They all involve making an indirect measurement of jig bed characteristics combined primarily with feedback control of the discharge of dense mineral from the jig, or less commonly, with manipulation of the jig operating parameters.
Most commonly, a so-called "float" is suspended in the bed by a vertical rod, or similar arrangement and the position of the float is sensed by electro-mechanical means. The float is usually a suitably shaped (e.g. "streamlined") body which, by use of weights can be caused to have a chosen or adjustable effective specific gravity. The float is usually intended to indicate the position of the top of the layer of most dense mineral in the bed. By maintaining the position of the top of the latter layer constant through regulation of discharge of the most dense mineral layer, it is intended that the SG.sub.50 for the jig shall remain constant.
In addition to the use of floats, it is also known to use pressure sensors to indicate the hydrostatic pressure at one or more points in the jig bed. The pressure signals can be interpreted to indicate the average specific gravity of the bed as a whole or the depth of the bed or the average specific gravity of the bed in a chosen zone of the bed.
In the control of bed depth or specific gravity, it must be recognised that the jig operates in a cyclical way due to the regular pulsation of the fluid in the jig. The periodic motion of the fluid results in corresponding periodic variations in the jig bed properties. Consequently, the measures of float position or pressures must be made at a prescribed point in time within the jig cycle or period, or the signal from the sensor must be averaged over the jig cycle in a meaningful way.
It is also known to use signals from pressure sensors located in the jig bed, water level indicators or mechanical paddle sensors to assist in jig regulation (e.g. see British Pat. No. 1,597,231 (Norton-Harty Colliers Engineering Limited) and German Pat. No. 1,217,292 (Stamicarbon NV)). The signals from the sensors at prescribed times within the jig cycle or as average values are interpreted to indicate the general condition of the jig bed. The signals from mechanical paddles (torque signal) can be interpreted as an indication of the degreee of bed expansion caused by the jig pulsion stroke. Regulation of jig discharge or jig stroke can be employed to maintain signals indicative of general bed properties constant.
The most direct measure of jig bed density known is described by Bartelt (D. Bartelt, "Regulating Jig Discharge by means of Radioisotopes", Fourth International Coal Preparation Congress, 1962, Paper B-2, pp. 89-97). Bartelt employed a gamma ray source (Caesium 137) and a radiation detector (halogen-quenched Geiger counter tube) to determine the average jig bed density at a chosen horizon in the jig bed. This technique of measurement significantly improved regulation of jig bed properties and the jig separation efficiency when the measurement signal was employed to regulate jig bed discharge instead of a float sensor signal.
The first Addition to French Pat. No. 1,382,798 (Beteiligungs-und Patentverwaltungs (GmbH) describes a method for regulation of the jig bed discharge based simply on the mean absorption of the radiation, as a measure of bed density, in a specific horizontal plane in the bed, while German Pat. No. 1,115,651 (Maschinenfabrik Buckau R. Wolf AG) describes a method where the radiation source and detector are moved vertically to maintain a constant absorption rate, the movement being utilized to control the vertical position of the discharge gate to maintain the gate within a prescribed transition zone.
German Pat. No. 1,245,281 (Beteiligungs-und Patentverwaltungs GmbH) describes a method of controlling the discharge where the radiation absorption is only monitored during that portion of the cycle when the jig bed is densely packed. This method does recognise that the bed density in a particular horizontal plane varies with time within a jig cycle but fails to recognise that this density variation with time can be employed to measure the dilation of the bed and that bed dilation behaviour is important in establishing stratification.
German Pat. No. 1,123,631 (Mannesmann AG) describes a method for the continuous monitoring of the bed density to control the operation of the discharge gate on the ampblade of the water column, while German Pat. No. 1,131,611 (also by Mannesmann AG) describes a jig separator where the discharge gate or valve is opened when the absorption rate, and thereby the bed density, varies by a predetermined value from a present value.
German Pat. No. 1,132,872 (Mannesmann AG), which is a Patent of Addition to DE Pat. No. 1,123,631, uses two radiation detectors which are spaced vertically to enable a thicker transition zone to be monitored, the discharge gate being opened to discharge more material when the difference between the absorption measurement by the two detectors decreases, indicating an increase in the thickness of the transition zone.
German Pat. No. 1,140,881 (Mannesman AG) is a further Patent of Addition to DE Pat. No. 1,123,631 and discloses an arrangement of the jig separator for fine or medium granular material where a pair of detectors are provided adjacent the discharge gate, with the source in the middle of the bed.
(The methods described in DE Pat. Nos. 1,123,631, 1,131,611, 1,132,872 and 1,140,881 are also included in U.S. Pat. No. 3,082,873 of Bartelt.)